Deployable stent

ABSTRACT

A stent  1  comprising a sheet  2  of biocompatible material arranged as a tube and having a pattern of folds allowing the stent to be collapsed for deployment. The pattern of folds comprises a unit cell repeated over the sheet  2 . A variety of different unit cells  3  are described. The pattern of folds may progress helically around the tube which assists in implantation by synchronising deployment. The stent  1  prevents restenosis because it is formed of a continuous sheet  2 , and allows slippage to be minimised.

[0001] The present invention relates to a stent. The present provides anovel structure for a stent, and also relates to the manufacture and useof the stent.

[0002] A stent is a medical device designed to open up a blocked lumenat a site in the human (or even animal) body, for instance a coronaryartery, aorta or the oesophagus etc. The occlusion might be caused forinstance by a disease such as stenosis or by cancer. Stents preferablyhave a flexible structure allowing them to be collapsed to reduce theirouter dimensions. This is to facilitate the passage of the stent intothe site in the body where the stent is expanded for deployment. Typicaluses of a stent are to open blocked coronary arteries and large veins,to treat obstructions to breathing in the trachea and bronchus, to allowthe passage of urine in the prostate and, more recently, to palliatecancer stenosis in the oesophagus. Stent therapy is now widely acceptedfor interventional treatment not only in the vascular system, but alsothe gastrointestinal, belier and urinary systems. Stent techniques havecome to be regarded as simply, safe and effective in comparison to othersurgical or non-surgical treatments.

[0003] Known stents have one of five basic constructions that istubular, coil, ring, multi-design and mesh structures. Tubular stentsare rigid. The other types of known structures are collapsible andtypically comprise an open tubular structure of structural elementswhich may be collapsed to facilitate deployment. The various knownstructures have different features and advantages, for example a highexpansion rate, a high strength, good flexibility and/or goodtractability. Whilst some structures provide different combinations ofthese advantages, an ideal stent sharing all these advantages has yet tobe realised.

[0004] One of the major problems with known stents is restenosisoccurring after implantation. This is a particular problem for meshstents and other open structures as tissues grow through the stent andblock the lumen again and is a particular problem in oesophagealapplications. Some reports suggest that restenosis is due to cell damageoccurring during deployment at the blocked site as the stent pushesagainst the cell wall. The amount of such damage is dependent on thestent configuration. After significant tissue growth through a stent,the stent cannot be retrieved. Thus it may be necessary to implantfurther stents after a first stent becomes blocked in order to reopenthe blockage. As this involves stents being implanted inside oneanother, there is a limit to the number of stents which can be implantedat one location.

[0005] To overcome this problem, covered stents have been developed.Covered stents were developed by attaching a tubular flexible cover, forexample of polyester, attached around the outside of a wire mesh stentstructure. The use of such a cover around a wire mesh stent is aneffective way to prevent restenosis. However, the common problems ofcovered stents include rupture of the cover and, when used in theoesophagus, for example, bolus obstruction, tumour in-growth,gastro-oesophageal reflux, migration/slippage, and difficulties indelivery especially for high oesophaegeal malignancies and angulatedcardio-oesophageal lesions. The risk of slippage and hence migration ofthe stent is a particular problem. Such covered stents still rely, forexample, on a mesh frame for collapse and expansion during deployment,but there has been very little investigation of the integrated expandingmechanism when the stent is covered.

[0006] As a result of the problems described above for both covered anduncovered stents re-intervention is often required. As a result manypatients have sub-optimal response to this type of treatment.

[0007] Current expandable stents are expensive to manufacture due totheir complicated structures which are labourious to form. The high costhas reduced their widespread use. Particularly for the oesophagus,doctors often opt for cheaper, semi-rigid plastic tubular stents eventhough they carry a higher risk of oesophagus perforation than theexpandable stents.

[0008] The present invention is intended to provide a stent which avoidsat least some of the problems discussed above.

[0009] According to the present invention, there is provided a stentcomprising a biocompatible sheet arranged as a tube folded with apattern of folds allowing the stent to be collapsed for deployment.

[0010] Such a structure for a stent provides numerous advantages. As thestent is formed from a sheet, restenosis is prevented. Furthermore, thepattern of folds allows the stent to be collapsed for deploymentfacilitating delivery to the blocked site in the body. The pattern offolds may allow the tube to be collapsed both radially andlongitudinally of the tube. Alternatively, the pattern of folds may onlyallow the tube to be collapsed radially. Longitudinal collapse isadvantageous where the blocked site is particularly inaccessible. On theother hand, in many uses the medical practitioner finds it moreconvenient if there is no longitudinal collapse in order to judge thelength of the deployed stent prior to deployment. For such uses, it ispreferred to use a pattern of folds which has a reduced or zerolongitudinal collapse.

[0011] The stent may also reduce slippage as compared to a known coveredstent. Firstly, the folds may provide an uneven outer surface whichreduces slippage. Secondly, the outer surface may be provided with ahigh degree of friction, for example by selection of the biocompatiblematerial of the stent or by roughening the outer surface.

[0012] The use of a pattern of folds to collapse the stent allows it tobe packaged compactly and to have good flexibility for ease of deliveryto the blocked site. The structure can be simple in structural form andis hingeless which increases reliability. The design of the stent isgeneric, so it can be adapted for use at different anatomical sites. Forexample, by varying the diameter, length and/or bifurcation the stentmay be collapsed for retrieval at a later date after implantation.

[0013] As the stent may be manufactured simply by folding a sheet, it isrelatively simple to manufacture. This will minimise costs and it isanticipated that costs might be kept below many known expandable stents.

[0014] The stent is particularly useful for use in the oesophagus, whererestenosis is a particular problem, or as a stent graft in the aorta.However, the stent may be used at any site in the body by appropriatesizing of the stent.

[0015] Many different patterns of folds are possible. Desirably, forease of the design and manufacture wherein the pattern of foldscomprises a unit cell repeated over at least a portion of the sheet, orthe entire sheet.

[0016] In the preferred pattern of folds, wherein the unit cellcomprises: an outer circumferential edge of hill folds comprising a pairof longitudinal edge folds extending along the tube and transverse edgefolds extending around the tube; a central longitudinal fold extendingalong the tube between the transverse edge folds; and angular foldsextending from each intersection of a longitudinal edge fold with atransverse edge fold to the central longitudinal fold. Many variationswithin this pattern are possible.

[0017] The choice of pattern may be selected to balance the ease ofdeployment, which generally improves as the degree of overlap in thefolded pattern decreases, with the compactness of the stent whencollapsed, which generally improves as the degree of overlap in thefolded structure increases.

[0018] A particularly advantageous pattern is one which progresseshelically around the tube. For example, the pattern of folds may includeuninterrupted lines of folds progressing helically around the tube.Alternatively, the pattern of folds may comprise at least one row ofunit cells progressing helically around the tube.

[0019] Such a helical pattern provides a number of advantages. Firstly,it can be folded more compactly in the longitudinal direction, becauseof the twist in the pattern, in comparison with a stent having a patternof folds which does not progress helically.

[0020] Secondly, the stent with a helical pattern may be deployed moreeasily, because the collapse and expansion of the stent is synchronised.That is to say, the helical pattern causes the forces during collapseand expansion to be transferred along the tube. This may be envisaged asbeing caused by the force being transferred along uninterrupted lines offolds progressing helically around the tube. Consequently, collapse andexpansion of the tube may be induced by the simple expedient of twistingthe tube. As such a twist is easy to apply, this greatly improves theease of deployment.

[0021] Thirdly, the helical pattern holds the stent in its expandedconfiguration.

[0022] In order that the present invention may be better understood, thefollowing description of embodiments of the present invention is givenby way of non-limitative example with reference to the accompanyingdrawings, in which:

[0023]FIG. 1 is a perspective view of a stent using one of the foldingpatterns in accordance with the present invention;

[0024]FIG. 2 is a diagram of a unit cell of a pattern of folds with thesheet in its unfolded state;

[0025]FIG. 3 is a diagram of the sheet with unit cell of FIG. 2developed to form the overall pattern of folds with the sheet in itsunfolded state;

[0026]FIG. 4 is a progression of end views of the stent of FIG. 1 duringexpansion and contraction;

[0027]FIGS. 5, 7, 12, 13, 15-19, 21-24, 26-31, 33 and 34 are diagrams ofunit cells with alternative patterns of folds with the sheet in itsunfolded state;

[0028]FIGS. 6, 8, 14, 20, 25, 32, and 35-42 are diagrams of sheets withalternative patterns of folds with the sheet in its unfolded state;

[0029] FIGS. 9 to 11 are graphs of the change in dimensions of a stentagainst the number of unit cells in the pattern of folds; and

[0030]FIG. 43 is a view of a portion of a sheet of the stent showing anaperture at a node where folds intersect.

[0031] A stent 1 in accordance with the present invention is illustratedin FIG. 1. The stent 1 comprises a biocompatible sheet 2. The sheet 2 isarranged as a tube with a pattern of folds which allow the stent to becollapsed for deployment.

[0032] The pattern of folds comprises a unit cell 3 which is repeatedover the entire area of the sheet 2. The pattern of folds is illustratedmore clearly in FIGS. 2 and 3 which are views of, respectively, the unitcell 3 and the unit cells developed over the sheet 2 in the unfoldedstate, notionally “unwrapped” from its tubular form, the lines a-a andb-b being the same line longitudinally along the tube. The unit cells 3are in rows repeating around a direction perpendicular to thelongitudinal axis of the tube.

[0033] In FIGS. 2 and 3, and indeed the further figures illustratingpatterns of folds, the lines are fold lines where the sheet 2 is folded.Between the folds, the sheet 2 is flat or planar. Continuous and dashedlines indicate folds of first and second opposite types. The two typesare valley and hill folds. Hill folds are folds which form a peak whenviewed from the outer side of the tube. Valley folds are folds whichform a valley when viewed from the outer side of the tube. In thefollowing description, it will be assumed that the folds of the firsttype are hill folds and the folds of the second type are valley folds.

[0034] In general, the two types of fold are reversible in any givenpattern, that is replacing all hill folds with valley folds andreplacing all valley folds with hill folds. However, some patterns whenreversed cause the tube to lock and hence do not allow the tube to becollapsed or expanded. The present invention contemplates thealternative that the folds of the first type are valley folds and thefolds of the second type are hill folds, except when this causes lockingof the structure.

[0035] For convenience, the pattern of folds illustrated in FIGS. 1 to 3is referred to as Pattern 1.

[0036] The unit cell 3 comprises the following folds.

[0037] Unit cell 3 has an outer circumferential edge of hill folds. Inparticular, these are a pair of longitudinal edge folds 4 extendingalong the tube parallel to one another and transverse edge folds 5extending around the tube.

[0038] The unit cell 3 further comprises a central longitudinal fold 6extending along the tube between the transverse edge folds 5.

[0039] Lastly, the unit cell 3 has four angular folds 7 each extendingfrom a respective intersection A, C, D or F of a longitudinal edge fold4 with a transverse edge fold 5 to the central longitudinal fold 6. Allfour angular edge folds 7 intersect the central longitudinal fold 6 atthe same position O. The length l of each transverse edge fold 5, thatis from the intersection (e.g. at A) with a longitudinal edge fold 4 toa central intersection (e.g at B) with the central longitudinal fold 6,is equal to the length of the portion of the central longitudinal fold 6from the central intersection (e.g. at B) with the transverse edge fold5 to the intersection (e.g. at O) with an angular fold 7. Therefore, thetriangle AOB and equivalent triangles within the unit cells 3 areisosceles triangles. The angle α (e.g. angle OAB) between a transverseedge fold 5 and an angular fold 7 is 45°.

[0040] The unit cell 3 is symmetrical about the central longitudinalfold 6 and about an imaginary line extending around the tubeperpendicular to the central longitudinal fold 6 and intersecting thecentral longitudinal fold 6 at O.

[0041] The angular folds 7 are valley folds and the central longitudinalfold 6 is a hill fold. Accordingly, the unit cell 3 is folded asillustrated in perspective view in FIG. 1 where the intersections A to Fof the various fold lines from FIG. 2 are indicated for one of the unitcells 3.

[0042] The unit cell 3 is repeated as illustrated in FIG. 3. Inparticular, the unit cells 3 are arranged in rows 8 labelled n₁, n₂, . .. , the rows repeating along the tube. The unit cells 3 of adjacent rowsare offset, as illustrated by the unit cells 3 illustrated in boldoutline in FIG. 3, that is with the longitudinal edge folds 4 of eachrow 8 meeting the central longitudinal folds 6 of the adjacent rows 8.The number n of rows 8 labelled n₁, n₂, . . . in FIGS. 1 and 3 and thenumber m of unit cells 3 within each row around the tube labelled m₁,m₂, . . . in FIGS. 1 and 3 can be freely varied. Similarly, the absolutedimensions of the sheet 2 and the unit cell 3 can be freely varied.

[0043] One of the interesting properties of Pattern 1 is that it causesthe stent 1 to collapse and expand both longitudinally and radially.That is both the length of the tube and the radius of the tube increaseduring expansion and decrease during collapse. This property providesthe advantage that the folded stent 1 can be packaged compactly. Thismakes the stent 1 easier to deliver through narrow passages of the bodyand facilitates deployment at a blocked site where it can be expanded.

[0044]FIG. 4 is a progression of end views of the stent 1 during itsexpansion and contraction. As can be seen from FIG. 4, the central partof the unit cell 3 at the intersection (at O) of the angular fold 7 withthe central longitudinal fold 6 moves inwardly and outwardly, causing achange in the radius of the stent 1 during deployment. This also causesa reduction in the distance between the intersections (at B and E)between the central longitudinal fold 6 and the transverse edge folds 5,which causes a change in the axial length L of the stent 1.

[0045] Further possible patterns of folds will now be described. Thefurther patterns of folds are variations on Pattern 1 shown in FIGS. 1to 3. For clarity and for brevity, the further patterns will all bedescribed by explaining the variations from Pattern 1 without repeatingthe common features. The same reference numerals as for Pattern 1 willbe used to denote the sheet 2, the unit cell 3, the equivalent folds 4to 7 and the rows 8.

[0046] Pattern 2 is illustrated in FIGS. 5 and 6. FIG. 5 is a diagram ofthe unit cell 3 and FIG. 6 is a diagram of the sheet with the unit cell3 developed across the sheet 2. Pattern 2 is similar to Pattern 1 exceptthat the angle a (e.g. angle OAB) between each transverse edge fold 5and angular fold 7 is less than 45°, so the unit cell 3 is no longerrectangular.

[0047] Pattern 3 is illustrated in FIGS. 7 and 8. FIG. 7 is a diagram ofthe unit cell 3 and FIG. 8 is a diagram of the sheet 2 with the unitcell 3 developed across the sheet 2.

[0048] Pattern 3 varies from Pattern 1 in that the angle a (e.g. angleOAB) between each transverse edge fold 5 and in respect of angular fold7 is greater than 45° and less than or equal to 60°. As a result theshape of the unit shape 3 becomes a polygon. The angle a should be equalto or less than 60° to allow folding of the sheet 2.

[0049] Pattern 3 also varies from Pattern 1 in that the angular folds 7do not all intersect the central longitudinal fold 6 at the sameposition. Instead, for each pair of angular folds 7 at oppositelongitudinal ends of the unit cell 3, the pair of angular folds 7intersect the central longitudinal folds 6 at the same position, but thepairs of angular folds 7 intersect the central longitudinal fold 6 atseparated positions O and X. Between these separated positions O and X,the central longitudinal fold 6 is a valley fold. However, the portionsof the central longitudinal fold 6 extending from a central intersection(at B or E) with a respective one of the transverse edge folds 5 to arespective intersection (at O or X) with the angular folds 7 remain ashill folds. The separation between the intersections (at O and X) ofeach pair of angular folds 7 and the central longitudinal fold 6 may befreely varied. This separation may be reduced to zero (as in Patterns 1and 2), but the longitudinal length of the unit cell 3, or moreparticularly the length of the central longitudinal fold 6, may not befurther reduced or else folding is prevented.

[0050] To understand and compare the folding of Patterns 1 to 3, thegeometric properties of Patterns 1 to 3 have been analysed as follows.The analysis is based on Pattern 2 with the angle a as 30° and onPattern 3 with the angle a as 60°.

[0051] Firstly, the ratio R* of the outer radius of stent 1 (ie thedistance from Oo to A or B) in its fully folded configuration to theouter radius of the stent 1 in its fully deployed configuration wascalculated for stents 1 having differing numbers m of unit cells 3 ineach row 8 of the sheet 2 around the tube. The relationship between R*and m for Patterns 1, 2 and 3 is illustrated in FIG. 9 where Pattern 1is shown by a continuous line, Pattern 2 is shown by a dotted line andPattern 3 is shown by a dashed line.

[0052] For each model, it will be noted that the value of R* decreasesas the number m of unit cells 3 in each row 8 increases. In other words,a large value of m makes the pattern fold more compact in the radialdirection. Thus the number m of unit cells 3 in each row 8 around thetube is preferably large to minimise the radius of the stent 1 oncollapse. However, increasing the number m of unit cells 3 in each row 8causes the folding to become complex and potentially affected by thethickness of the material of the sheet 2. The number m of unit cells 3in each row 8 should be selected to balance these two factors.

[0053] It will also be noted from FIG. 9 that as compared to Pattern 1,Pattern 2 has a lower value of R* and hence folds more compactly,whereas Pattern 3 has a higher value of R* and hence folds lesscompactly. However, the difference in the value of R* between Patterns1, 2 and 3 becomes small when m is larger than 9. When m=10the radius ofthe stent 1 in its fully folded configuration is about 30% of that inits fully deployed configuration, for each pattern.

[0054] Also, the value L* of the ratio of the total length of the stent1 in its fully folded configuration to the length of the stent 1 in itsfully deployed configuration was calculated for different values of thenumber m of unit cells in each row 8 of the sheet 2 around the tube andfor differing values of the number n of rows 8 along the tube.

[0055]FIG. 10 shows the value of L* for each of Patterns 1 to 3 fordiffering values of n when m=6. In FIG. 10, Pattern 1 is shown by acontinuous line, the Pattern 2 is shown by a dotted line and Pattern 3is shown by a dashed line. It will be seen that for each model, theratio L* slowly decreases as n increases. This means that all threePatterns fold more compactly in the longitudinal direction as the numbern of rows 8 of unit cells 3 increases. The value of L* becomes nearlyconstant when n is greater than 7, so there is no particular benefit inincreasing the number n of unit cells 3 above about 7.

[0056] It will be noted that, as compared to Pattern 1 in thelongitudinal direction, Pattern 3 folds more compactly, whereas Pattern2 folds less compactly but maintains flexibility. Therefore, pattern 3is preferred for uses where longitudinal collapse is desirable to allowaccess of the stent 1 to the blocked site, whereas Pattern 2 ispreferred for uses where the medical practitioner prefers thelongitudinal collapse to be minimised.

[0057]FIG. 11 shows the value of L* for Pattern 1 for different valuesof m when n=7. It will be noted that L* becomes smaller as m increases.Thus increasing m reduces the longitudinal collapse of the stent 1 whenfolded, as well as reducing the radial collapse.

[0058]FIGS. 12 and 13 are diagrams of the unit cells 3 of the Patterns1-1 and 2-1 which are variations of Patterns 1 and 2, respectively. FIG.14 is a diagram of Pattern 1-1 developed across the sheet 2. In bothcases, the length of the unit cell 3 is increased so that the pairs ofangular folds 7 intersect the central longitudinal fold 6 at separatedpositions O and X between which the central longitudinal fold 6 is avalley fold.

[0059] FIGS. 15 to 19 are diagrams of the unit cell 3 of Patterns 1-2,2-2, 3-1, 1-3 and 2-3, respectively, which are themselves variations ofmodels 1, 2, 3, 1-1 and 2-1, respectively. FIG. 20 is a diagram ofPattern 1-2 with the unit cell 3 developed across the sheet of material2. In each case, the variation is to provide an additional ring ofvalley folds 9. Each valley fold 9 extends parallel to an adjacentlongitudinal or transverse edge fold 4 or 5. The valley folds 9 extendsbetween an angular fold 7 and either another angular fold 7 or else thecentral longitudinal fold 6. The ring of valley folds 9 causes thesurface of the unit cell 3 to be folded twice. Therefore inside the ringof valley folds 9, the folds of the basic unit cell 3, that is theangular fold 7 and the central longitudinal fold 6, reverse. That is tosay, hill folds reverse to valley folds and valley folds reverse to hillfolds. Such a ring of valley folds 9 has the advantages that the doublefolding pattern causes the inner surface of the sheet 2 inside the tubeto become smoother and allows the unit cell 3 to be folded morecompactly, because the peak point O of the unit cell 3 in its foldedstate shown in FIG. 4 is folded inside points A and C of the folded unitcell 3, ie allowing the unit cells 3 to be folded compactly in theradial direction.

[0060] The unit cells 3 described above are symmetrical both about thecentral longitudinal fold 6 and also about an imaginary line extendingaround the tube perpendicular to the central longitudinal fold 6.However, this is not essential. Either or both degrees of symmetry maybe removed. For example FIGS. 21 to 24 are diagrams of Patterns 4-1 to4-4, respectively, which are symmetrical only about the centrallongitudinal fold 6. FIG. 25 is a diagram of Pattern 4-1 with the unitcell 3 developed over the sheet 2. Accordingly, the unit cell 3 ofalternate rows 8 is reversed in the longitudinal direction. This mayalso be viewed as a Pattern having a larger unit cell comprising the twounit cells 3 illustrated in FIG. 21 in bold outline combined together.Patterns 4-1 to 4-4 may also be viewed as consisting of the other halfof one of the Patterns described above with the lower of another of thePatterns described above. For example, Pattern 4-1 may be viewed as theupper half of Pattern 1 combined with the lower half of Pattern 2, andso on.

[0061] FIGS. 26 to 29 are diagrams of the unit cell 3 of Patterns 5-1 to5-4, respectively, which are variations of Patterns 4-1 to 4-4,respectively, the variation is that the unit cell 3 further comprises aring of valley folds 9 as in Patterns 1-2, 2-2, 3-1, 1-3 and 2-3.

[0062]FIGS. 30 and 31 illustrate Patterns 6-1 and 6-2 which aresymmetrical only about an imaginary line extending around the tube.These Patterns may also be viewed as combinations oflongitudinally-extending halves of different Patterns described above,except that the central longitudinal fold 6 extends at an angle to thelongitudinal direction along which the longitudinal edge folds 4 extend.In particular, if the angle BAO is α₁, and then the angle BCO is α₂,then the angle AOB is α₂, the angle BOC is α₁, and both angles ABO andCBO are (π−α₁ −α₂). For example, Pattern 6-1 may be viewed as thecombination of the left half of Pattern 1 with the right half of Pattern2. Similarly, Pattern 6-2 may be viewed as the combination of the lefthalf of Pattern 2-1 and the right half of Pattern 3.

[0063] Unlike the previous patterns, Pattern 6-2 cannot be used byitself, but must be combined with another pattern. For example, FIG. 32is a diagram of Pattern 6-2 with the unit cell 3 developed over a sheet2 and combined with Pattern 3. To enable the unit cells to fit together,alternate unit cells 3 of Pattern 6-2 along each row 8 arelongitudinally reversed and a unit cell of Pattern 3 is arranged betweensuccessive pairs of unit cells 3 of Pattern 6-2, between the longerlongitudinal edges of the unit cells 3 of Pattern 6-2. Thus, a largerunit cell is formed by the combination of two unit cells 3 of Pattern6-2 with a unit cell of Pattern 3.

[0064]FIGS. 33 and 34 are diagrams of the unit cell 3 of Patterns 7-1and 7-2 which are variations of Patterns 6-1 and 6-2. The variation isthe addition of a ring of valley folds 9 similar to the valley folds 9of Patterns 1-2, 2-2, 3-1, 1-3 and 2-3.

[0065] In the Patterns described above, a single unit cell 3 is repeatedover the entire sheet, but this is not essential. In fact, differentunit cells 3 may be repeated over different portions of the sheet 2. Forexample, FIGS. 35 to 39 show patterns of folds in which different rows 8comprise a respective, different unit cell 3 repeated around the tube.In FIGS. 35 and 36, two different patterns are used. In FIG. 35,Patterns 1 and 1-1 are used for alternate rows. In FIG. 36, Patterns 1and 1-2 are used for alternate rows. In FIGS. 37 and 38, three differentpatterns are used. In particular, in both FIGS. 37 and 38 unit cells 3of Patterns 1, 4-1 and 2 are used for different respective rows 8,although in a different order longitudinally along the tube.

[0066] Similarly, FIG. 39 is a diagram of a pattern of folds in whicheach row 8 comprises two different unit cells 3 alternating along therow 8, in particular the unit cells of Patterns 1 and 1-2.

[0067] The patterns of folds described above produce a tube which isgenerally cylindrical by means of the unit cells 3 being arranged withparallel longitudinal edge folds 4 and has the same radius along thelength of the tube. However, this is not essential. For example, thesheet 2 may be arranged in a tube which is conical along the entirelength or along a portion thereof. This may be achieved using thepattern of folds illustrated in FIG. 40 which is based on a unit cell 3of Pattern 2, but in which the unit cells 3 are of different sizes withthe longitudinal edge folds 4 being angled relative to one another,instead of parallel. Therefore the longitudinal edge folds 4 are alsoangled with respect to the longitudinal direction of the tube. As aresult, the sheet 2 of FIG. 40 forms a conical (or frustoconical) tubewhen folded. Alternatively, the tube may have a more complicatedstructure, for example having plural tubular portions branching off froma common node.

[0068]FIGS. 41 and 42 are diagrams of patterns in which unit cells 3 arearranged on the sheet 2 in rows 8 which progress helically around thetube when the sheet 2 is folded. FIGS. 41 and 42 are based on a unitcell of Pattern 1, but any of the patterns described above couldalternatively be used. Consequently, the rows 8 of unit cells arearranged at a pitch angle or helix angle β which is the angle betweenthe direction in which the unit cells repeat and plane perpendicular tothe longitudinal axis of the tube. When the sheet 2 is folded with theopposite lines a-a and b-b in FIG. 41 and 42 being the same line,successive rows 8 of unit cells 3 join end-to-end to form a longer rowwhich progresses helically around the tube. In the pattern of FIG. 41,the angle × is selected so that the rows 8 combine to form a single rowprogressing helically around the tube. In the pattern of FIG. 42, theangle β is selected so that the rows 8 join together to form two rowsprogressing helically around the tube.

[0069] As a result of the helical pattern it will also be noted that thelongitudinal edge folds 4 and the central longitudinal folds 6 ofalternate rows 8 meet together to form an uninterrupted fold line whichalso progresses helically around the tube.

[0070] Such a helical structure provides a number of advantages.Firstly, it allows the sheet 2 to be folded compactly in thelongitudinal direction because of its capability of torsion. Secondly,the helical pattern assists with deployment, because the expansion andcollapse of the sheet 2 is usually synchronised over the area of thesheet 2. That is to say, the helical progression of the pattern of foldsspreads the force causing expansion or collapse to be transmitted alongthe length of the tube. This may be viewed as the force beingtransmitted along the uninterrupted lines of folds formed by thelongitudinal edge folds 4 and the central longitudinal folds 6 ofalternate rows 2 which progress helically around the tube. This meansthat a twist applied to the stent 1 can be used to generate expansion orcollapse of the stent 1 which greatly assists deployment because a twistis simple to perform. Thirdly, the helical structure holds the stent 1in its expanded configuration. This is because collapse of the stentrequires torsional forces which are not usually developed at sites inthe body.

[0071] The patterns described above are preferred because of theirsimplicity and hence ease of design and manufacture. However a stent inaccordance with the present invention may be formed using numerous otherpatterns of folds which allow radial collapse and optionallylongitudinal collapse. Alternative patterns may be regular or irregularand the sheet between the folds may in general be flat or curved.

[0072] The stents described above may be combined together to form alarger product or may have additional components added thereto.

[0073] Manufacture of a stent 1 in accordance with the present inventionwill now be described.

[0074] First, the biocompatible sheet 2 is provided.

[0075] The sheet may initially be planar, in which case opposed edges ofthe sheet are subsequently joined together to form a tube. In this case,in the drawings, the lines a-a and b-b may represent edges of the sheet2 which are joined together. Alternatively, the sheet 2 may bemanufactured be formed as a tube ab initio, that is with the sheet beingcontinuous around the tube. In this case, in the drawing, the lines a-aand b-b are the same imaginary line along the length of the tube. Thislatter alternative has the advantage of avoiding the need to join theedges of a planar sheet but makes it harder to form the folds.

[0076] In its most simple form, the sheet 2 is a sheet of biocompatiblematerial. Any biocompatible material may be used. Generally, abiocompatible material is selected with appropriate mechanicalproperties for the site at which the stent 1 is to be used. The materialshould be selected to be sufficiently rigid to hold its shape betweenthe folds when implanted in a lumen. This is to perform its basicfunction of holding the lumen open. This must be balanced against theease of folding the sheet and the need for the collapsed stent 1 to besufficiently flexible to allow delivery to the blocked site.

[0077] Suitable materials for the sheet 2 include stainless steel or ashape memory alloy such as Nitinol. It is also anticipated that manypolymers would be suitable.

[0078] The sheet 2 is desirably selected so that the outer surface ofthe sheet 2 on the outside of the tube provides a sufficient degree offriction to provide anchorage at the anatomical site where it is to beimplanted. This may be achieved by selecting a material providing anappropriate degree of friction or by roughening the outer surface.

[0079] Alternatively, the sheet 2 may be a multi-layer material. In thatcase, the inner and outer layers may be selected to provide appropriatedegrees of friction. Desirably the outer surface of the sheet 2 on theoutside of the tube provides a higher degree of friction than the innersurface of the sheet 2 of the inner side of the tube.

[0080] In another form, the sheet 2 may have a coating of abiocompatible material.

[0081] The sheet 2 is folded with the desired pattern of folds. This maybe achieved by initially forming fold lines which facilitate subsequentfolding. For example, the fold lines may be formed by scoring the sheetor by stamping the sheet from both sides by stamps having ridges alongthe fold lines, the stamp on one side having the pattern of hill foldsthe other side having the pattern of valley folds.

[0082] However the preferred techniques to form fold lines are laserlithography and chemical etching.

[0083] In the case of laser lithography, a laser is used to form scoresin the surface of the sheet 2 along the fold lines. The laser equipmentfor such processing is in itself conventional.

[0084] In the case of chemical etching, the sheet 2 is first masked by amaterial resistant to a chemical enchant, except along the desiredpattern of folds. Then the etchant is applied to the sheet to etchscores in the pattern of folds where the masking material is notpresent. Subsequently the masking material is removed. Such a chemicaletching process in itself is conventional. Preferably, a conventionalphotolithographic technique is used. In such a case, the maskingmaterial is a positive or negative photoresist applied across the entiresheet. Ultra-violet light is applied in an image of the pattern offolds, being positive or negative image for the case of a positive ornegative photoresist, respectively. This alters the photoresist allowingit to be removed by the etchant in the pattern of folds, but leaving itresistant to the etchant elsewhere.

[0085] In general, the etchant and the masking material may be chosenhaving regard to the material of the sheet 2. However, particularpossibilities are as follows.

[0086] In the case of a chemical etching of a sheet 2 of stainlesssteel, one possibility is to use the negative etching technique commonlyused for etching stainless steel, for example using ferric chloride and1% HCl as the etchant and using a dry film as a negative photoresist.

[0087] In the case of chemical etching of a sheet 2 of shape memoryalloy, the following positive etching technique has been applied using apositive photoresist layer of solid contented HRP 504 or 506 as themasking material and using a mixture of hydrofluoric and nitric acid asthe etchant. The etching method was applied to a sheet 2 of thickness 80μm and size 80 mm×80 mm which was cleaned to improve the adhesion of themasking material. The masking material was applied by dip coating at aspeed of 6 mm/min to create a coating of thickness 12 μm. The sheet 2was then soft-baked at 75° for 30 minutes. The masking material was thenexposed by UV light with a positive image of the pattern of folds onboth sides of the sheet, and the image developed using PLS1: H₂O=1:4.Finally the sheet 2 was hard-baked at 120° for 60 minutes. The sheet 2was then etched using a mixture of hydrofluoric acid and nitric acid inproportions HF 10%, HNO₃ 40%, H₂O 50% or HF:HNO₃:H₂O=1:1:2 or 1:1:4.

[0088] Other ways to chemically etch a sheet 2 of shape memory alloy arenegative etching with a rubber-type of photoresist or electrochemicaletching with H₂SO4/CH₃OH, for example as disclosed in Eiji Makino, etal., “Electrochemical Photoetching of Rolled Shape Memory Alloy Sheetsfor Microactuators”, Vol. 49, No. 8, 1998; and D M Allen, “ThePrinciples and Practice of Photochemical Machining and Photoetching”,Adam Hilger, 1986.

[0089] In the case of a sheet 2 which is initially planar, after foldingthe edges of the folded sheet 2 are joined together to form the sheet 2into a tube.

[0090] As the stent is folded from a sheet 2, it is possible tomanufacture it relatively cheaply. It is anticipated that manufacturingcosts may be kept below current costs of many existing expandablestents.

[0091] The dimensions of the sheet 2, the type of pattern of folds andthe dimensions of the unit cell 3 within the pattern of folds areselected based on the site at which the stent is intended to be used.The stent 1 may be used for treatment at sites in any type of lumen inthe body simply by choice the dimensions and mechanical properties ofthe sheet of the stent 1. Once deployed, the stent 1 preventsrestenosis, because it is formed from a sheet 2 which is effectivelycontinuous. The stent 1 is particularly advantageous for use in theoesophagus where restenosis is a particular problem.

[0092] The stent 1 is used in the same manner as known stents, that isby initially collapsing the stent 1 to deliver the stent 1 to the siteto be treated and subsequently expanding the stent 1. Manipulation ofthe stent 1 is performed using conventional medical techniques.

[0093] A potential problem with the stent 1 as described above is thathigh stresses are developed at the nodes where the folds 4 to 7intersect. Such stresses could create weakness at the nodes, potentiallycausing sheet 2 to puncture or rip. To avoid this problem, aperture 10may be formed in the sheet 2 at the nodes where the folds 4 and 7intersect, or at least at those nodes where high stresses are likely tobe developed.

[0094] An example of such an aperture 10 formed in a sheet 2 at the nodewhere the longitudinal edge folds 4, the transverse edge folds 5 and theangular fold 7 intersect is shown in FIG. 43. The aperture 10 in FIG. 43is shown as being circular, but may be of any shape. The aperture 10 issufficiently small that it does not allow significant in-growth throughthe aperture 10, hence effectively retaining the continuous nature ofthe sheet 2.

1. A stent comprising a biocompatible sheet arranged as a tube foldedwith a pattern of folds allowing the stent to be collapsed fordeployment.
 2. A stent according to claim 1, wherein the pattern offolds comprises a unit cell repeated over at least a portion of thesheet.
 3. A stent according to claim 2, wherein the unit cell comprises:an outer circumferential edge of folds of a first type comprising a pairof longitudinal edge folds extending along the tube and transverse edgefolds extending around the tube; a central longitudinal fold extendingalong the tube between the transverse edge folds; and angular foldsextending from each intersection of a longitudinal edge fold with atransverse edge fold to the central longitudinal fold.
 4. A stentaccording to claim 3, wherein for each pair of angular folds at oppositelongitudinal ends of the unit cell, the pair of angular folds intersectthe central longitudinal fold at the same position.
 5. A stent accordingto claim 4, wherein the lengths of: a transverse edge fold from theintersection with a longitudinal edge fold to a central intersectionwith the central longitudinal fold; and the portion of the centrallongitudinal fold from said central intersection with the transverseedge fold to the intersection with the angular folds, are the same.
 6. Astent according to any one of claims 3 to 5, wherein the angular foldsare folds of a second type inverse to said first type and the portionsof the central longitudinal fold extending from a respective one of thetransverse edge folds to a respective intersection with the angularfolds are folds of said first type.
 7. A stent according to any one ofclaims 3 to 6, wherein the angular folds all intersect the centrallongitudinal fold at the same position
 8. A stent according to any oneof claims 3 to 6, wherein said pairs of angular folds intersect thecentral longitudinal fold at separated positions between which thecentral longitudinal fold is a fold of said second type.
 9. A stentaccording to any one of claims 3 to 8, wherein the angle between atransverse edge fold and an angular fold is 45°.
 10. A stent accordingto any one of claims 3 to 8, wherein the angle between a transverse edgefold and an angular fold is less than 45°.
 11. A stent according to anyone of claims 3 to 8, wherein the angle between a transverse edge foldand an angular fold is greater than 45° and less than or equal to 60°.12. A stent according to any one of claims 3 to 11, wherein the unitcell further comprises a ring of folds of said second type each parallelto an adjacent edge fold, the pattern of folds inside the ring of foldsof said second type reversing from folds of said first type to folds ofsaid second type and vice versa.
 13. A stent according to any one ofclaims 3 to 12, wherein the unit cell is symmetrical about the centrallongitudinal fold.
 14. A stent according to any one of claims 3 to 13,wherein the unit cell is symmetrical about an imaginary line extendingaround the tube.
 15. A stent according to any one of claims 3 to 13,wherein the folds of said first type are hill folds and the folds ofsaid second type are valley folds.
 16. A stent according to any one ofclaims 2 to 15, wherein the pattern of folds comprises a single unitcell repeated over the entire sheet.
 17. A stent according to any one ofclaims 2 to 16, wherein the pattern of folds comprises a plurality ofrows of unit cells extending around the tube.
 18. A stent according toclaim 17 when appendant to claim 3, wherein the unit cells of adjacentrows are offset with the longitudinal edge folds of each row meeting thecentral longitudinal folds of the adjacent rows.
 19. A stent accordingto claim 18, wherein the longitudinal edge folds of alternate rows andthe central longitudinal folds which meet form an uninterrupted foldline progressing helically around the tube.
 20. A stent according to anyone of claims 17 to 19, wherein at least two of the rows comprise arespective, different unit cell repeated around the tube.
 21. A stentaccording to any one of claims 2 to 15, wherein the pattern of foldscomprises at least one row of unit cells progressing helically aroundthe tube.
 22. A stent according to claim 22, wherein the pattern offolds comprises two rows of unit cells each two rows progressinghelically around the tube.
 23. A stent according to any one of thepreceding claims, wherein the pattern of folds includes uninterruptedlines of folds progressing helically around the tube.
 24. A stentaccording to any one of the preceding claims, wherein the tube isconical along at least a portion thereof.
 25. A stent according to anyone of the preceding claims, wherein the sheet has apertures at at leastsome of the nodes where folds intersect.
 26. A stent according to anyone of the preceding claims, wherein the outer surface of the sheet onthe outer side of the tube has a higher degree of friction than theinner surface of the sheet on the inner side of the tube.
 27. A stentaccording to any one of the preceding claims, wherein the outer surfaceof the sheet on the outer side of the tube is roughened.
 28. A stentaccording to any one of the preceding claims wherein the outer surfaceof the sheet on the outer side of the tube has a sufficient degree offriction to provide anchorage at an anatomical site.
 29. A stentaccording to any one of the preceding claims, wherein the sheet hasedges joined along the length of the tube.
 30. A stent according to anyone of claims 1 to 28, wherein the sheet is continuous around the tube.31. Use of a stent according to any one of the preceding claims.
 32. Aplanar sheet of biocompatible material having a pattern of fold linesarranged to allow the sheet to be folded into a tube to form a stentwhich is collapsible for deployment.
 33. A method of manufacturing astent comprising folding a planar sheet of biocompatible material into atube with a pattern of folds allowing the stent to be collapsed fordeployment.
 34. A method of manufacturing a stent comprising folding asheet of biocompatable material in the form of a tube with a pattern offolds allowing the stent to be collapsed for deployment.